I need help understanding AC electricity and generators (1/3 phase)

[strantor]

I can't fix this video, but I'll say one thing, that's the most knowledgeable appliance repairman I've ever seen. I don't think I've ever met an appliance repairman who even knows how to use a scope, much less owns one. I would hire him on the spot.

Here's another video, explaining how to make a ground-referenced scope measure across L1-L2. Add & invert on ch2. I wonder why he needs to invert?

Oscilloscope operation differential voltage measurement - YouTube

(See 2:50)

 

[strantor]

I've fixed the Wikipedia page. For anyone who is curious, this is what it used to say:



Here's what it says now:

 

[LouNY]

It's getting deep on this thread,

 

[CalG]

You are playing fast and free with "ground" which is actually neutral.

If you measure L1 to N on one scope channel and N to L2 on the other, of course it will appear both are in phase. This isn't valid because you are measuring from two different references.

Your battery analogy is incorrect because both batteries are always positive on their positive terminal. L1 is negative when L2 is positive. L1 and L2 always have N in the middle. You can not parallel L1 and L2, they are hard wired in series.

Your CRT scope has channel "add" functions to add or subtract Channel 1 from Channel 2 specifically so one can connect 1 to L1 and 2 to L2 to see the combined signal while each channel's ground is connected to neutral.

Don't confuse ground with neutral. For safety they are kept close but they are not equal.

Your transformers have phase dots specifically because the phases are 180ｰ apart.

L1 to L2 is a single in-sync phase.
L1 to N and L2 to N are 180ｰ out of phase.
L1 to N and N to L2 are in phase but separated by 170 volts.

Yeay! Grumpycat!

 

[strantor]

I think the problem here is everyone's (including mine) own personal familiarity with electrical power distribution. If you're willing & able, divorce the issue from the electrical grid and look at the physics.

Here's the simplest precalculus/trig video I could find on the topic of adding sinusoidal waves. Don't take it from me, take it from a teacher:

2 combining waves - YouTube

Waves existing in the same medium (any waves, water waves, sound waves, trig waves on a graphing calculator, electrical waves, etc.) of equal magnitude and frequency, in phase, add to make a wave of the same frequency but double the amplitude. These same waves, if 180 degrees out of phase, cancel each other out. Zero amplitude. This is how sound canceling headphones work.

Electrical waves do not violate the laws of physics. If they add and the result is a wave of the same frequency but double the amplitude, then they have to be in phase. No way around that. If your oscilloscope disagrees, it's because you're feeding it one inverted wave and one non-inverted wave. In order to make your oscilloscope calculate the correct double-amplitude (240v) wave, you must use the add and invert function together. I dont know what else to say.

 

[MossRoad]

Can you find anything anywhere that supports your position, because EVERY article I've found on the subject says they are 180 degrees out of phase.

 

[Mat4644]

Waves existing in the same medium (any waves, water waves, sound waves, trig waves on a graphing calculator, electrical waves, etc.) of equal magnitude and frequency, in phase, add to make a wave of the same frequency but double the amplitude. These same waves, if 180 degrees out of phase, cancel each other out. Zero amplitude. This is how sound canceling headphones work.

Electrical waves do not violate the laws of physics. If they add and the result is a wave of the same frequency but double the amplitude, then they have to be in phase. No way around that. If your oscilloscope disagrees, it's because you're feeding it one inverted wave and one non-inverted wave. In order to make your oscilloscope calculate the correct double-amplitude (240v) wave, you must use the add and invert function together. I dont know what else to say.

All of these examples are not describing the output from a center tap transformer. The key to understanding this is to study how it operates and the waveforms produced. It's single phase and L1 and L2 in relationship ONLY to each other are 180 degrees apart as measured to the center tap.

 

[CalG]

All of these examples are not describing the output from a center tap transformer. The key to understanding this is to study how it operates and the waveforms produced. It's single phase and L1 and L2 in relationship ONLY to each other are 180 degrees apart as measured to the center tap.

Of course, the original question had to do with using two legs of a three phase generator to produce 220/240 volt for home use.

Oh the complexity!

 

[MossRoad]

One more try.... :laughing:

electrical - Are power outlets in a home out of phase with each other? - Home Improvement Stack Exchange

 

[strantor]

Can you find anything anywhere that supports your position

Of course not.

because EVERY article I've found on the subject says they are 180 degrees out of phase.

Because spouting off "180 degrees out of phase" is the convention, just like saying "current flows from positive to negative." Nobody would contradict that on their employer's letterhead. Not if this thread is any indication of the reception that truth would receive.

Those who actually know what they're talking about and aren't comfortable with saying nonsense things just because everyone else is, will word it more carefully, like here:

https://www.transcat.com/media/pdf/Power_meas.pdf

"In this system the two "hot" or current carrying conductors are 180 degrees out-of-phase with respect to the neutral."

A fair compromise I suppose; it's technically correct and doesn't ruffle many feathers. Sounds just like the expected rote "L1 is 180 degrees out of phase with L2" mantra but it isn't. It makes the important distinction.

If you find someone willing to acknowledge that L1 is only out of phase with L2 with respect to neutral, then the conversation is only about 10 words away from the word "semantics."

But this is not an issue of semantics. Reference points do not reverse the direction of current flow. We can talk about Reference points until we are blue in the face and it won't change this (substitue batteries for transformer windings if necessary):



When you say "L1 is 180 degrees out of phase with L2," you are saying that it literally is. But it isn't. It's only appears out of phase when using ground as a reference point. That's an important distinction that very few people are willing to make. I dont know why.